I'm dual-booting Windows 10 and Manjaro 17 and when I installed the latter I forgot to add a swap partition.

Noticed that and tried to create one from GParted but it is returning the following error when I click on 'New':

It is not possible to create more than 4 primary partitions

If you want more partitions you should first create an extended
partition. Such a partition can contain other partitions. Because an
extended partition is also a primary partition it might be necessary
to remove a primary partition first.

What Error do you get from gparted when trying this?
– SteseAug 16 '17 at 15:06

Quoted it below: "It is not possible to create more than 4 primary partitions If you want more partitions you should first create an extended partition. Such a partition can contain other partitions. Because an extended partition is also a primary partition it might be necessary to remove a primary partition first." Can't add images for some reason.
– Alexandru BuradaAug 16 '17 at 15:08

You should do as directed: "You should first create an extended partition." this requires you to delete a partition then recreate the partition you deleted and create the partition for the swap. You should delete dev/sda4 in order to accomplish this.
– RamhoundAug 16 '17 at 15:10

It may be possible to convert one primary partition to a logical partition using fixparts. See this page for details, and in particular the "Adjusting Your Partitions" section.
– Rod SmithAug 16 '17 at 16:36